End-to-end connections of a data transmission system, such as a telephone network, often show long transit time delays, in consequence of which echo is observed, for instance, in the case of normal speech, when a signal is reflected from the far end of a connection back to the talker. An echo canceller is an analog or digital device for processing a signal, such as a speech signal, which device is used for reducing echo by subtracting estimated echo from the echo (signal) occurring in a connection.
However, echo cancellation cannot be used in all cases, as for instance:
1) in connection with establishing a speech connection during multifrequency signalling (DTMF) in a telephone channel. Simultaneously transmitted signals produce a double talk situation and operation of an echo canceller would cause undesired changes in the signalling. PA1 2) during a data transmission effected as a continuous bit flow in a telephone channel, because the bit flow must get through unchanged at original transfer rate. PA1 3) during a data transmission effected by means of a baseband modem in a telephone channel. PA1 4) during automatic testings and measurings of a speech connection. For accurate results, the echo cancellation must be switched off. PA1 5) If there are several echo cancelling devices in a speech connection, only the echo cancellers at the ends of the connection are used and operation of the intermediate echo cancellers is disabled.
In order that echo cancellers along a connection can be switched on and off in appropriate situations, they must be capable of monitoring channel-associated or common channel signalling to observe the commands relating to the echo cancelling. In the case of channel-associated signalling, polarities of bits a and c of line signalling (T16) can be used to control the echo cancellet. Also, in common channel signalling, particular signals have been reserved for the control of the echo canceller, but for the identification of the signals, the echo cancellet must be provided with a common channel signalling terminal. It will be especially difficult to control the echo cancellet, if a common channel signalling time slot is routed along a path different from that of the speech itself.